The invention relates to satellite communications systems generally, and more particularly to satellite communication systems which divide the transmitted signal, either in power or in content, to be synchronized and recombined in the receiving terminal. This concept applies readily to broadcast applications, but it not so limited.
The satellite industry has experienced a progression of performance enhancements evidenced by increased transmit power capability of satellite transponders, improved low-noise amplifier (LNA) characteristics, and smaller receiving antennas. In satellite systems with a large number of receiving stations, it is particularly important to reduce the cost of each receiving unit and to design a system with a small receiving antenna to meet installation and aesthetic requirements. The need for a small receiving antenna has motivated an increase in transponder power output in order to maintain an acceptable signal-to-noise ratio (SNR) with the smaller antenna. As satellite users move from lower power transponders to higher power transponders, falling demand for the lower power transponders reduces the cost of their use. Receiving a signal from a lower power transponder with the smaller receiving antenna size produces a received power at the LNA too low to maintain SNR requirements. The present invention permits the receiver to combine received signals from a plurality of transponders, possibly located on a plurality of satellites to enable again the use of lower power transponders, but with small receiving terminal antennas.
A satellite communications system includes a transmitting station that directs information-carrying signals toward an orbiting satellite, which receives the signals and in turn retransmits the signals on a different frequency band toward a plurality of receiving terminals. The satellite contains a transponder which receives signals as a broad band of frequencies and retransmits them on another set of frequencies of equal bandwidth but shifted to another location in the spectrum.
The present invention has as its object a satellite communications system including a transmitting facility that divides the signal into a plurality of subchannels directed toward a plurality of transponders located on one or more satellites and a receiving terminal that receives the subchannels, time-synchronizes the subchannels, and combines them into a faithful replica of the original composite signal. The division of the signal into subchannels is carried out by one of two methods. In a first division method, the source signal is replicated across the plurality of transponders. Hereinafter the first division method is referred to as power-division. In a second division method, the content of the source signal is represented by a set of distinct subsignals, each of which subsignals contains less information as the original signal, but said distinct subsignals can be conveniently recombined in the receiver to reconstruct the original signal satisfactorily. Hereinafter this second division method is referred to as content-division.
In a system using power-division to create subchannels, the originating transmitter directs more than one identical signal to a plurality of transponders, possibly located on a plurality of satellites. In said system, transponders retransmit and the receiving antenna system conducts all of the aforementioned signals into the receiving system. The receiving terminal provides means of time-synchronizing the plurality of received signals, adjusts the relative power level of the plurality of received signals to be approximately equal, and combines the signals into a composite via a signal adding process. Each of the signals added contains both an information component and a random noise component, such noise having been introduced primarily in the first amplifier of the receiver, typically a low-noise block converter (LNB). Those skilled in the art know that the information component of each signal will be statistically correlated, but the noise components will be statistically uncorrelated, both to each other and to the information component. Thus the information components will add linearly into the composite signal, that is in proportion to their number. The power in the information component of the composite signal will then be in proportion to the square of the number of received signals being added together. In contrast, the power in the noise component of the composite signal will be in proportion to the number of received signals added together. Thus the SNR of the composite signal is improved over the SNR of the individual subchannel signals by a factor of N in power, where N is the number of channels added together to form the composite signal. The foregoing discussion assumes that the signal levels and noise levels in each of the subchannel signals is identical.
In a real system, transmission characteristics will vary slightly between subchannels, signal and noise levels being slightly different between subchannels, resulting in an SNR improvement ratio somewhat less than the factor of N described above. In any case, the receiver may require automatic means of adjusting the power of each of the signals to be added at the combining point so as to be approximately equal to each other in level.
In a system using content-division to create subchannels, the originating transmitter directs distinct subsignals toward the plurality of transponders, the subsignals being created in such a way as to permit convenient reconstruction of the original signal at the receiving terminal. In an exemplary analog system, the original signal can be divided into subband signals using a filter-bank process. If the filters used satisfy quadrature-mirror properties, the subsignals can be added directly to reproduce the original signal without phase distortion at the boundary frequencies. If the analog signal contains a strong periodic timing component (as does a television signal), this periodic timing component can be separated from the remainder of the signal before dividing the signal into subband components. Said timing component could then be added back to each of the subband components to produce subchannel signals with different frequency components, but common timing information. This strategy naturally provides timing information useful to facilitate the necessary time-resynchronizing process in the receiver.
As above, in a system using content-division to create subchannels, the originating transmitter directs distinct subsignals toward the plurality of transponders, the subsignals being created in such a way as to permit convenient reconstruction of the original signal at the receiving terminal. In an exemplary digital system, the original binary signal can be divided into subchannel digital signals, each of which has a bit rate less than the original digital signal. The original digital signal can be divided into subchannel digital signals in any number of ways. Two simple exemplary digital subchannel strategies are as follows. A first exemplary digital subchannel strategy is to direct each successive bit into each subchannel sequentially. A second exemplary digital subchannel strategy is to direct each fixed-size block of bits in the original signal to each successive subchannel sequentially. This second exemplary strategy fits well with digital source signals that are organized in a fixed-block-size structure in the original signal.
In the case that a plurality of satellites is used to conduct a set of subchannels from a transmitting station to a given receiving terminal, each subchannel will generally experience a different propagation delay. The instant invention provides means to determine the amount of time to delay each subchannel in order to combine them synchronously. The delay required for each received subchannel will in the general case differ. The present invention provides additional means to implement the aforedetermined delay for each subchannel independently.
The receiving terminal system, when activated for a particular virtual channel, determines the relative delay between the subchannel signals arriving at the receiver. This could be accomplished by correlating the subchannel signals with each other at all possible delays expected in a particular implementation of the system. As this process is very time consuming and source signal dependent, it is therefore subject to false synchronization and possible failure to synchronize at all, particularly if the source signal does not contain enough timing information. The present invention solves this problem by transmitting a timing signal along with the original signal. Said timing signal arrives at the receiving terminal via a plurality of propagation paths, each experiencing a different delay. The timing signal is added to the virtual satellite system in such a way so as to be separable from the original signal on each subchannel. The receiving terminal then correlates timing signals arriving on different subchannels to determine the amount of relative propagation delay. All subchannel signals contain common timing information to facilitate the correlation process. This guarantees that the subchannels can be processed and compared in a known way to determine relative propagation delay.
The timing signal can be added to the virtual satellite channel using one of two exemplary methods, but the instant invention is not so limited. A first exemplary method requires that a narrow bandwidth signal be transmitted across each satellite in the virtual channel. The narrow band signal requires a small allocation of the available spectrum, but provides a dedicated timing signal on each satellite actively carrying virtual satellite channels. The narrow band timing signal provides propagation delay information to virtual channel receiving terminals having one or more subchannels on the satellite. The timing signal could consist of one or more of the following exemplary signals, but the instant invention is not so limited. A first exemplary signal is a carrier modulated digitally by a binary pseudorandom noise sequence. A second exemplary signal is a periodic pulse. The pulse could be time-dispersed prior to transmission to create a signal with improved peak to average waveform properties. The receiving terminal in this example would reverse the time-dispersal process to recover a narrow-time pulse. The time period of either exemplary signal above described, after which the signal repeats, would be longer than twice the greatest expected delay difference between subchannels, thus facilitating unambiguous determination of propagation delay.
A second exemplary method of incorporating a timing signal in the virtual satellite system consists of adding a spread spectrum component to each of the information-bearing subchannels in the system, and within the bandwidth of each subchannel. The magnitude of the spread spectrum timing component is much lower than the information signal so as not to reduce the performance of the normal receiver demodulation process. The spread spectrum signal is then despread in the receiving terminal, thereby increasing its magnitude above that of the information content. The increase in signal level is proportional to the processing gain. This process facilitates delay synchronization in the receiving terminal and has two advantages. A first advantage is that the second exemplary method does not increase the bandwidth requirements of the virtual channel to accommodate a timing signal. A second advantage is that the full bandwidth of the information channel is available to the timing signal resulting in high resolution relative delay estimation.